Optical fiber

ABSTRACT

Optical fiber capable of controlling attenuation losses caused by microbending on the signal transmitted thereby. The optical fiber comprises: a) an internal glass portion; b) a first coating layer of a first polymeric material surrounding said glass portion; and c) a second coating layer of a second polymeric material surrounding said first coating layer, wherein said first polymeric material has a hardening temperature lower than −10° C., an equilibrium tensile modulus lower than 1.5 MPa, wherein said first coating layer has a thickness of from 18 μm to 28 μm and wherein said second coating layer has a thickness of from 10 μm to 20 μm.

This is a continuation of U.S. patent application Ser. No. 10/512,238,filed Jun. 3, 2005, now allowed, which was a national phase applicationbased on PCT/EP02/04507, filed Apr. 24, 2002, all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical fiber, in particular to anoptical fiber capable of controlling the attenuation losses caused bymicrobending on the signal transmitted thereby.

BACKGROUND ART

Optical fibers commonly consist of a glass portion (typically with adiameter of about 120-130 μm), inside which the transmitted opticalsignal is confined, The glass portion is typically protected by an outercoating, typically of polymeric material. This protective coatingtypically comprises a first coating layer positioned directly onto theglass surface, also known as the “primary coating,” and of at least asecond coating layer, also known as “secondary coating,” disposed tosurround said first coating. In the art, the combination of primarycoating and secondary coating is sometimes also identified as “primarycoating system,” as both these layer are generally applied during thedrawing manufacturing process of the fiber, in contrast with “secondarycoating layers” which may be applied subsequently. In this case, thecoating in contact with the glass portion of the fiber is called “innerprimary coating” while the coating on the outer surface of the fiber iscalled “outer primary coating.” In the present description and claims,the two coating layers will be identified as primary and secondarycoating, respectively, and the combination of the two as “coatingsystem.”

The thickness of the primary coating typically ranges from about 25 μmto about 35 μm, while the thickness of the secondary coating typicallyranges from about 10 μm to about 30 μm.

These polymer coatings may be obtained from compositions comprisingoligomers and monomers that are generally crosslinked by means of UVirradiation in the presence of a suitable photo-initiator. The twocoatings described above differ, inter alia, in the mechanicalproperties of the respective materials. As a matter of fact, whereas thematerial which forms the primary coating is a relatively soft material,with a relatively low modulus of elasticity at room temperature, thematerial which forms the secondary coating is relatively harder, havinghigher modulus of elasticity values at room temperature. The coatingsystem is selected to provide environmental protection to the glassfiber and resistance, inter alia, to the well-known phenomenon ofmicrobending, which can lead to attenuation of the signal transmissioncapability of the fiber and is therefore undesirable. In addition,coating system is designed to provide the desired resistance to physicalhandling forces, such as those encountered when the fiber is submittedto cabling operations.

The optical fiber thus composed usually has a total diameter of about250 μm. However, for particular applications, this total diameter mayalso be smaller; in this case, a coating of reduced thickness isgenerally applied.

In addition, as the operator must be able to identify different fiberswith certainty when a plurality of fibers are contained in the samehousing, it is convenient to color the various fibers with differentidentifying colors. Typically, an optical fiber is color-identified bysurrounding the secondary coating with a third colored polymer layer,commonly known as “ink”, having a thickness typically of between about 2μm and about 10 μm, or alternatively by introducing a colored pigmentdirectly into the composition of the secondary coating.

Among the parameters which characterize primary and secondary coatingsperformances, elastic modulus and glass transition temperature of thecross-linked materials are those which are generally used to define themechanical properties of the coating. When referring to the elasticmodulus it should be clarified that in the patent literature this issometimes referred to as “shear” modulus (or modulus measured in shear),while in some other cases as “tensile” modulus (or modulus measured intension). The determination of said elastic moduli can be made by meansof DMA (Dynamic mechanical analysis) which is a thermal analysistechnique that measures the properties of the materials as they aredeformed under periodical stress. For polymeric materials, the ratiobetween the two moduli is generally 1:3, i.e. the tensile modulus of apolymeric material is typically about three times the shear modulus (seefor instance the reference book Mechanical Properties and Testing ofPolymers, pp. 183-186; Ed. G. M. Swallowe)

Examples of coating systems are disclosed, for instance, in U.S. Pat.No. 4,962,992. In said patent, it is stated that a soft primary coatingis more likely to resist to lateral loading and thus to microbending. Itthus teaches that an equilibrium shear modulus of about 70-200 psi(0.48-1.38 MPa) is acceptable, while it is preferred that such modulusbeing of 70-150 psi (0.48-1.03 MPa). These values correspond to atensile modulus E′ of 1.4-4.13 MPa and 1.4-3.1 MPa, respectively. Asdisclosed in said patent, a too low equilibrium modulus may cause fiberbuckling inside the primary coating and delamination of the coatingsystem. In addition, said patents suggests that the glass transitiontemperature (Tg) of the primary coating material should not exceed −40°C., said Tg being defined as the temperature, determined by means ofstress/strain measurement, at which the modulus of the material changesfrom a relatively high value occurring in the lower temperature, glassystate of the material to a lower value occurring in the transitionregion to the higher temperature, elastomeric (or rubbery) state of thematerial.

However, as noticed by the Applicants, although a primary coating has arelatively low value of Tg (as generally required by the art), the valueof the modulus of the coating material may nevertheless begin toincrease at temperatures much higher than the Tg, typically alreadyabove 0° C. Thus, while a low value of Tg simply implies that thetransition of said coating from its rubbery to its glassy state takesplace at relatively low temperatures, no information can be derived asto which would be the variation of the modulus upon temperaturedecrease. As a matter of fact, an excessive increase of the modulus ofthe primary coating upon temperature decrease may negatively affect theoptical performances of the optical fiber, in particular at the lowtemperature values, thus causing undesirable attenuation of thetransmitted signal due to microbending.

This problem is further worsened when the optical fibers are insertedinto a cable structure, typically within a polymeric protecting sheath,which may in general take the form of a tube. Microbending typicallyarises whenever the optical fibers get in contact with the surface ofsaid housing sheath. For instance, as the coefficient of thermalexpansion of polymeric materials generally employed as protectingsheaths is much higher than the one of glass, upon temperature decreasethe polymeric sheath is thus subjected to a greater shrinkage withrespect to optical fibers. This results in the optical fibers to becomein contact with the inner walls of the tube, thus possibly determining alocal pressure which may then result in the microbending phenomena.

Thus, as observed by the Applicants, what seems important forcontrolling the microbending of an optical fiber, particularly wheninserted into a cable structure, is the temperature at which the coatingmaterial begins the transition from its rubbery state (soft) to itsglassy state (hard), which temperature will be referred in the followingof this specification and claims as the “hardening temperature” of thematerial, or Th. In addition, the Applicants have observed that themicrobending of an optical fiber can be further controlled by using aprimary coating with a relatively low equilibrium modulus and by using aprimary coating and a second coating layer having specific thicknessranges. Particularly advantageous results are obtained by selecting acured composition which still shows a relatively low modulus at said Th,so that an excessive increase of the modulus upon further temperaturedecrease is avoided.

In the present description and claims, the term “modulus” is referred tothe modulus of a polymeric material as determined by means of a DMA testin tension, as illustrated in detail in the test method section of theexperimental part of the present specification.

In the present description and claims, the term “hardening temperature”is referred to the transition temperature at which the material shows anappreciable increase of its modulus (upon temperature decrease), thusindicating the beginning of an appreciable change from a relatively softand flexible material (rubber-like material) into a relatively hard andbrittle material (glass-like material). The mathematical determinationof Th will be explained in detail in the following of the description.

According to the present invention, the Applicants have thus found thatattenuation losses caused by microbending onto a coated optical fibers,particularly at the low exercise temperatures, can be reduced bysuitably controlling the increase of the modulus at the lowtemperatures. In particular, the Applicants have found that saidmicrobending losses can be reduced by using a polymeric material for theprimary coating having a low hardening temperature and by using aprimary coating and a second coating layer having specific thicknessranges. In addition, the Applicants have found that by selecting coatingcompositions having a relatively low equilibrium modulus, saidattenuation losses can be further controlled over the whole operatingtemperature range.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to an opticalfiber comprising: a) an internal glass portion; b) a first coating layerof a first polymeric material surrounding said glass portion; and c) asecond coating layer of a second polymeric material surrounding saidfirst coating layer, wherein said first polymeric material has ahardening temperature lower than −10° C., an equilibrium tensile moduluslower than 1.5 MPa, wherein said first coating layer has a thickness offrom 18 μm to 28 μm and wherein said second coating layer has athickness of from 10 μm to 20 μm.

Preferably, the equilibrium modulus of said first polymeric material islower than 1.4 MPa and more preferably lower than 1.3 MPa.

According to a preferred embodiment, the polymeric material forming saidprimary coating has:

-   -   a) a hardening temperature (Th) of from −10° C. to about −20° C.        and a modulus measured at said Th lower than 5.0 MPa; or    -   b) a hardening temperature (Th) of from −20° C. to about −30° C.        and a modulus measured at said Th lower than 20.0 MPa; or    -   c) a hardening temperature (Th) lower than about −30° C. and a        modulus measured at said Th lower than 70.0 MPa.

Preferably said material forming said coating layer has:

-   -   a) a hardening temperature (Th) of from −10° C. to about −20° C.        and a modulus measured at said Th lower than 4.0 MPa; or    -   b) a hardening temperature (Th) of from −20° C. to about −30° C.        and a modulus measured at said Th lower than 15.0 MPa; or    -   a hardening temperature (Th) lower than about −30° C. and a        modulus measured at said Th lower than 50.0 MPa.

According to a further preferred embodiment the glass transitiontemperature of the material is not higher than about −30° C., morepreferably not higher than −40° C. and much more preferably not higherthan −50° C.

Preferably, said first polymeric material is obtained by curing aradiation curable composition comprising a radiation curable oligomercomprising a backbone derived from polypropylene glycol and a dimer acidbased polyester polyol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of an optical fiber;

FIG. 2 shows an illustrative DMA plot of a polymeric material suitablefor an optical fiber according to the invention;

FIG. 3 shows the curve corresponding to the first derivative of the DMAplot of FIG. 2;

FIG. 4 a to 4 c show the experimental DMA plots of three primary coatingmaterials suitable for an optical fiber according to the invention;

FIG. 5 shows the experimental DMA plot of a prior art primary coatingmaterial.

FIG. 6 shows an illustrative embodiment of a drawing tower formanufacturing an optical fiber.

FIGS. 7 to 9 shows different examples of optical cables.

DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned above, the optical fiber according to the inventioncomprises on a glass portion thereof a primary coating layer formed froma polymeric material having a relatively low hardening temperature andlow equilibrium modulus.

To better explain the meaning of the hardening temperature, reference ismade to the curve shown in FIG. 1. This curve, typically obtained by aDMA (Dynamic Mechanical Analysis), represents the variation of themodulus of a polymeric material vs. temperature. As shown by this curve,the polymeric material has a relatively high value of modulus at the lowtemperatures (glassy state, portion “a” of the curve), while said valuebecomes much lower when the polymer is in its rubbery state, at thehigher temperatures (portion “b” of the curve, equilibrium modulus). Theoblique portion “d” of the curve represents the transition of thematerial from the glassy to the rubbery state. The transition betweenthe glassy state and the rubbery state is known in the art as the “glasstransition” of the material and is generally associated to a specifictemperature (Tg, glass transition temperature). As apparent from thecurve, the transition between the glassy and the rubbery state takesplace over a relatively wide range of temperatures. For apparentpractical reasons, methods has thus been developed for determining aspecific Tg value for each polymer. One of this methods (see forinstance P. Haines, “Thermal Methods of Analysis,” p. 133. BlackieAcademic and professionals ed.), which is the one used for determiningthe Tg values indicated in the present description and claims, comprisesdetermining the intersection point of two lines. The first line(identified as “A” in FIG. 2) is determined by interpolating the pointsof the DMA curve in the plateau region of the glassy state (portion “a”of the curve). In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from −60° C. to−80° C. The second line (identified as “D” in FIG. 2) is determined asthe tangent to the inflection point of the DMA curve in the obliqueportion “d” of said curve. The inflection point and the inclination ofthe tangent in that point can be determined as usual by means of thefirst derivative of the DMA curve, as shown in FIG. 3. According to thecurve shown in FIG. 3, the abscissa of the minimum point of the curvegives the respective abscissa of the inflection point on the DMA curveof FIG. 2, while the ordinate gives the inclination (angularcoefficient) of the tangent line in said inflection point.

In the practice, the derivative of each experimental point is firstcalculated and then the curve interpolating the derivative points isdetermined as known in the art. For avoiding unnecessary calculations,only those points falling within a relatively narrow temperature rangearound the minimum point are taken into account for the regression.Depending from the distribution of the experimental points, this rangemay vary between 40° C. (about ±20° C. around the minimum point) and 60°C. (about ±20° C. around the minimum point). A 6^(th) degree polynomialcurve is considered in general sufficient to obtain an curve to fit withthe derivative of the experimental points.

As shown in FIG. 2 the so determined glass transition temperature is ofabout −62° C.

Similarly to the Tg, also the hardening temperature (Th) of a polymericmaterial can be determined by the above method. The Th is thusdetermined as the intersection point between line “B” and the abovedefined line “D”, as shown in FIG. 2. Line “B” is determined byinterpolating the points of the DMA curve in the plateau region of therubbery state (portion “b” of the curve) i.e. at the equilibrium modulusof the material. In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from 20° C. and40° C.

As shown in FIG. 2, the Th calculated according to the above method willthus be of about −13° C.

As observed by the applicants, when the cured material forming theprimary coating of the optical fiber has a Th lower than about −10° C.,preferably lower than −12° C. and an equilibrium modulus lower than 1.5MPa, preferably lower than 1.4 MPa and more preferably lower than 1.3MPa, the optical performance of the optical fiber can be furtherimproved, particularly by reducing its microbending sensitivity in thewhole operating temperature range and particularly at the lowtemperatures of exercise, e.g. below 0° C. As a matter of fact, thecombination of these two parameters in a cured polymeric materialapplied as primary coating on an optical fiber according to theinvention results in a relatively smooth increase of the modulus upontemperature decrease, thus allowing to control the microbendingphenomena down to the lower operating temperature limits, typically −30°C.

Said modulus should however preferably be not lower than about 0.5 MPa,more preferably not lower than about 0.8 MPa in order not to negativelyaffect other properties of the fiber, such as the adhesion of thecoating material to the glass portion of the fiber.

As further observed by the Applicants, when the cured material formingthe primary coating of the optical fiber has a Th lower than about −10°C. and a modulus lower than 5.0 MPa, preferably lower than about 4.0MPa, at said temperature, said control of the microbending phenomena canbe further improved.

An analogous improved control of the microbending phenomena can beachieved also when the cured polymeric material has a Th lower than −20°C. and a modulus at said temperature lower than 20 MPa, preferably lowerthan 15 MPa, or when the cured polymeric material has a Th lower than−30° C. and a modulus at said temperature lower than 70 MPa, preferablylower than 50 MPa.

Furthermore, the glass transition temperature of the cured polymericmaterial applied as primary coating on an optical fiber according to theinvention is preferably not higher than about −30° C., more preferablynot higher than −40° C. and much more preferably not higher than −50° C.

All the above indicated parameters, i.e. modulus, Th and Tg can bedetermined by subjecting a polymeric material to a DMA in tensionperformed according to the methodology illustrated in the experimentalpart of the present specification, and by evaluating the respective DMAplot of the material according to the above defined procedure.

A coating material according to the invention is particularlyadvantageous when applied to fiber design, whenever microbendingsensitivity is one of the main constraints in fiber design. This isespecially true for temperature below 0° C., as low as −30° C. Theimprovement in terms of lower microbending sensitivity provides thefiber designer with added margin, which could be exploited in severaldifferent manners.

For example, the added margin could increase the tolerances both offiber production, in terms of microbending sensitivity, and of cableproduction, in terms of stress over the fibers, without detrimentaleffects in added losses due to microbending.

Alternatively, or in addition, the added margin could be spent bydesigning special fibers which show an increased sensitivity tomicrobending, such as those illustrated hereinafter. Some of the opticalparameters of an optical fiber which may affect its microbendingsensitivity are the effective area and the refractive index profile (orα-profile). These parameters are well known to those skilled on the art.For instance, reference can be made to WO 01/49624, which describesthese and other parameters, also in relation to different kinds ofoptical fibers.

FIG. 1, illustrates an optical fiber comprising an internal glassportion 101, a first polymeric coating layer 102, also known as primarycoating, disposed to surround said glass portion and a second polymericcoating layer 103, also known as secondary coating, disposed to surroundsaid first polymeric layer.

A first example of advantageous application of the invention can befound in the use of a primary coating material as above described with avery large effective area fiber, having effective area at 1550 nmgreater than 90 μm² (as compared with an effective area of about 80 μm²for standard single mode fibers having a refractive index profile of thestep-index kind, i.e. a single segment profile, with a single variationof the refractive index of 0.2%-0.4%, a core radius of about 4.0-4.5 μmand a MAC value of about 7.8-8.6).

Fibers having effective area larger than 90 μm² are currently sold inthe market, for example with the commercial name of Vascade™ L1000 byCorning or UltraWave™ SLA by OFS or Z-plus Fiber™ by Sumitomo.

The effective area at 1550 nm for these kind of fibers is preferablyabove about 100 μm², more preferably above about 110 μm², even morepreferably above 120 μm².

Typically, these fibers have a zero dispersion wavelength between about1270 and 1330 nm and a positive dispersion slope at 1550 nm. Theattenuation at 1550 nm is advantageously below 0.200 dB/km, preferablybelow 0.190 dB/km. The refractive index profile commonly used is of thekind of step-index profile, i.e. a single segment profile. Fiber cutoffis between about 1250 nm and about 1650 nm, preferably between about1350 nm and about 1550 nm.

One of the main constraint in an attempt to design fibers with effectivearea greater than 90 μm² is the existence of a limit value formicrobending sensitivity above which the fiber could experience an addedloss in cable. As a matter of fact, the attenuation losses caused bymicrobending increases along with the increase of the value of theeffective area. It may thus be appreciated that this kind of opticalfibers is particularly advantageous for controlling these attenuationlosses.

Another example of advantageous application of the invention consists inthe combination of the above primary coating material with the so called“dispersion-shifted” optical fibers, characterized by an effective areaat 1550 nm greater than 60 μm² and a zero dispersion wavelength shiftedaway from the 1300 nm band.

Dispersion-shifted fibers having effective area larger than 60 μm² arecurrently sold in the market, for example with the commercial name ofFreeLight™ by Pirelli or TeraLight™ by Alcatei or Submarine LEAF® byCorning.

The effective area at 1550 nm is above about 60 μm² with preferred valueabove 70 μm², more preferably above about 80 μm².

Typically, these fibers have a zero dispersion wavelength between about1350 and 1650 nm and a positive dispersion slope at 1550 nm. Preferably,the zero dispersion wavelength is between about 1350 and 1520 nm. Theattenuation at 1550 nm is typically below 0.210 dB/km, preferably below0.200 dB/km.

Because of the relatively complex refractive index profile (as comparedto the conventional step-index profile of standard single mode opticalfibers), these fibers are more prone to attenuation losses caused bymicrobending and may thus take advantage from the optical fiber of theinvention.

A further example of advantageous application of the new materialconsists in the combination of the new coating material with adispersion compensating fiber (DCF).

Dispersion compensating fibers are currently sold in the market, forexample with the commercial name of Vascade™ S1000 by Corning orUltraWave™ IDF by OFS or Dispersion Compensating Modules by OFS.

Dispersion compensating fibers are suited to compensate the chromaticdispersion cumulated along the optical line having positive dispersion.They can be classified into two families; one for lumped compensation incompact modules and the other for distributed compensation in cableform.

Dispersion compensating fibers are characterized in that the dispersionat 1550 nm is below −20 ps/nm/km, preferably less than −30 ps/nm/km.

In the art, the chromatic dispersion first derivative with respect towavelength is called dispersion slope. In order to compensate also theslope of the transmission line, the dispersion slope of DCF ispreferably negative.

The first family of DCF is characterized in that the dispersion at 1550nm is below −80 ps/nm/km, preferably less than −100 ps/nm/km and morepreferably below −120 ps/nm/km.

The second family of DCF is characterized in that the dispersion at 1550nm is below −20 ps/nm/km, preferably less than −40 ps/nm/km and morepreferably below −60 ps/nm/km.

The effective area at 1550 nm is above about 15 μm², preferably aboveabout 20 μm², more preferably above 25 μm².

The refractive index profile preferably used for DCF fibers comprises acore and a cladding, wherein the core further comprises a centralsegment having positive refractive index difference with respect tocladding, a first annular segment having negative index difference and asecond annular segment having positive index difference.

As the microbending sensitivity of DCF typically increases with theincrease of the dispersion value, it may be appreciated that the fiberdesigner can in fact take advantage of the decreased microbendingsensitivity by lowering the dispersion of the DCF while keeping theremaining fiber optical properties substantially the same. Inparticular, for a given cutoff, effective area, dispersion to sloperatio, the fiber designer is allowed to decrease the dispersion at 1550nm while maintaining still acceptable microbending performances.

A further example of advantageous application of the invention is with amultimode optical fiber.

Multimode fibers are currently sold in the market, for example with thecommercial name of Infinicor® by Corning or GLight™ by Alcatel.

Multimode fibers found advantageous application in Local Area Networkand short reach link, typically below 1 km. The typical core diameter is50 μm and 65 μm.

Multimode fibers are strongly affected by the phenomenon of microbendingbecause of the mode coupling between the different propagation modes ofthe fiber. The fibers of the invention thus have increased resistance tolosses induced by microbending when placed in cable.

Space saving is becoming a crucial issue in telecommunicationapplications. For example, in the distribution network environment, theavailability of higher fiber count cables or reduced size cables wouldbe very advantageous to cope with the need of connecting great number ofusers with the constraint to use pre-existent ducts of various kind.Another example of advantage of a reduced diameter fiber andconsequently of reduced diameter cable can be represented by aerialcables. In this application, it is mandatory to reduce the overallweight of the cables and their wind resistance.

It should however be noted that while attempting to reduce overall fiberdimensions and weight, it is important to guarantee the fullcompatibility with existing fibers. Therefore the preferred solutionconsists in a fiber having a 125 μm diameter glass portion (i.e. similarto the one of conventional fibers), coated by coating layers havingreduced overall thickness, e.g. for an overall external diameter of lessthan or equal to 210 μm.

With reference to FIG. 1, the thickness of the primary coating layer 102is of from about 18 μm to 28 μm, preferably of about 22-23 μm while thethickness of the secondary coating 103 is of from about 10 μm to about20 μm, preferably of about 15 μm.

Due to the reduced thickness of protective polymeric layers, this kindof fiber is more prone to the effects of side pressures and thus toattenuation caused by microbending. By using a primary coating materialaccording to the invention, the fiber will thus show acceptablemicrobending losses also with such a reduction in the protective layer.

The optical fiber of the invention allows to reduce the attenuationlosses caused by microbending in an optical fiber disposed within acable structure, particularly within a polymeric buffer tube. Theoptical fibers are in fact typically housed in the buffer tubes with anexcess length with respect to the length of the tube, in order to avoidmechanical stresses on the optical fibers when the cable is pulled, e.g.during installation. Depending on the dimensions of the buffer tube andon the amount of optical fibers housed therein, said excess length mayvary between about 0.1% and 1%. In particular, when the fibers arehoused within a so-called “central loose tube,” said excess length istypically around 1%, to compensate the longitudinal elongation of thebuffer tube. For particular installations however (e.g. local network),where a high fiber count within the same buffer tube is required, saidexcess fiber length is lowered, e.g. to 0.5% or lower, to avoid possiblecontacts of the fibers with the internal walls of the tube. For theso-called “stranded loose tubes” structures, where a plurality of buffertubes is stranded (typically with a S-Z fashion) around a centralstrength member, the fiber excess length is generally lower, e.g.between 0.1% and 0.5%, as with this kind of structure the optical fibersare less prone to mechanical stresses when the cable is subjected tolongitudinal elongation.

In whichever case, attenuation losses may however arise upon temperaturedecrease, due to the higher shrinkage of the buffer tube with respect tothe glass fibers upon temperature decrease. In this case, an excessivemeandering of the fibers within the buffer tube will cause to fiber tocontact the inner walls of the buffer tube, with consequentmicrobending. In addition, particularly in the “stranded loose tubes”structure, or in those cases where the fibers are housed with a highfiber count within the buffer tube (i.e. with about 50% or more of theinternal cross area of the tube being occupied by the optical fibers) itmay happen that the fibers are forced towards the walls of the tube,e.g. as a consequence of a permanent bending of the cable in theinstallation duct. Also in this case, said contact may give rise tomicrobending. Similarly, optical fibers (e.g. in the form of opticalfiber ribbons) housed within a groove within the so-called “slottedcore” structure, may be forced either towards the outer sheath as aconsequence of temperature decrease or towards the bottom of the grooveas a consequence of a mechanical strain applied onto the cable.

Examples of cable structures wherein the use of said fibers can beadvantageous are illustrated in FIGS. 7, 8 and 9.

The cable shown in FIG. 7 has in its radially innermost position areinforcing element 701, typically made from glass-fiber reinforcedplastic, coated with a layer 702 of polymeric material, for instance apolyolefin, e.g. polyethylene or ethylene-propylene copolymer. The cablehas one or more polymeric tubular elements 703 (“buffer tubes”) whichcan be made from a polyolefin material (e.g. polyethylene orethylene-propylene copolymer) said tubes comprising a number of opticalfibers 704 which are embedded in a filling material 705, typically ofthe grease-like type (for instance as disclosed in U.S. Pat. No.6,278,824).

The optical fibers can be, for example, single-mode fibers, multi-modefibers, dispersion-shifted (DS) fibers, non-zero dispersion (NZD)fibers, or fibers with a large effective area and the like, depending onthe application requirements of the cable.

The number of tubular elements 703 present in the cable (which may alsobe arranged on several superposed layers) and the dimensions of thesetubular elements depend on the intended capacity of the cable, as wellas on the conditions under which this cable will be used. For example,six, eight or more tubular elements, arranged in one or more layers (forexample up to 48 tubes), can be disposed around the central element.

The tubular elements 703 are disposed in a helical lay around thecentral member, said lay being either a continuous helix or an openhelix obtained by alternate (S-Z) stranding of the tube. If desired, oneor more tubes may be replaced by one or more rods, in order to preservethe symmetry of the helical configuration in case the fiber count islower than the full fiber count. Alternatively, the central element canbe replaced by a further tubular element as those previously mentioned,apt to contain optical fibers.

The interstices 706 between the buffer tubes can also be filled with afilling compositions such as those previously mentioned or, preferably,with a composition having a higher viscosity.

Stranded tubes are generally bound together with a polymeric yarn ortape (not shown), e.g. a polyester or polypropylene yarn, in order toheld them firmly in their helical configuration during manufacturingprocesses.

A further polymeric tape (not shown) can be optionally wound withoverlapping around the stranded buffer tubes in order to allow aneffective containment of the interstitial water-blocking filler. Suchpolymeric tape, for instance polyester (e.g. Mylar®), has a thickness ofabout 25 to 50 μm and can be helical wound around the stranded buffertubes with an overlap of about 3 mm.

A water-blocking (or water swellable) tape 707 can be wound around thewhole structure. Such water-blocking tapes generally comprise apolymeric base tape on the surface of which a superabsorbent swellablematerial (e.g. polyacrylate or polymethylmethacrylate) in the form ofpowder is chemically or thermally fixed.

The stranded tubes can then be wrapped by a reinforcing layer 708, e.g.made of aramidic yarns (Kevlar®) or glass thread, optionally containingtwo sheath cutting threads 709 disposed longitudinally with respect tothe cable. An outer polymeric layer, e.g. is then disposed to surroundthe cable structure. Optionally, a metal tape (not shown), preferablycorrugated, can be disposed between the outer sheath 710 and thereinforcing layer.

The cable of FIG. 8 shows has in its radially innermost position areinforcing element 801 on which a polymeric slotted core 802 isextruded. Grooves 803 are formed longitudinally on the outer surface ofsaid core, which grooves extend either as a continuous helix or with anS-Z configuration along the whole outer surface of the said core. Thegrooves 803 are filled with a filler 804 as the one indicatedpreviously, and optical fibers in the form of ribbons 805 are embeddedtherein. The slotted core 802 is then wrapped by a containment tape 806,e.g. of polyester, surrounded by a waterbloking tape 807 as the oneindicated previously. A polymeric jacket 808, for instance ofpolyurethane or of a polyolefin material, is disposed to surround thewrapped slotted core. A reinforcing layer 809, e.g. made of aramidicyarns (Kevlar®) or glass thread, can be disposed to surround saidpolymeric sheath 808, optionally containing two sheath cutting threads(not shown) disposed longitudinally with respect to the cable. An outerpolymeric layer, is then disposed to surround the cable structure.Optionally, a metal tape 811, preferably corrugated, can be disposedbetween the outer sheath 810 and the reinforcing layer.

FIG. 9 shows a cross-sectional view of an optical fiber cable comprisinga central polymeric tube 901 (e.g. of polyolefin material), said tubecontaining a number of optical fibers 902 which are disposed loosely ina filling material 903 as previously mentioned. Groups of e.g. twelveoptical fibers can be grouped into sub-units and enveloped by a thinlayer of a low tensile modulus polymeric material (e.g.polyvinylchloride, ethylene-vinylacetate polymer, polyethylene orpolypropylene) to form a sub-module 904. The polymeric sheath can becolored in order to facilitate the identification of the fibers.

The number of optical elements 904 present (which may also be arrangedon several layers) and the dimensions of these elements depend on theintended capacity of the cable, as well as on the conditions under whichthis cable will be used. For example, both cables with a single opticalelement 904 and cables with six, eight or more optical elements,arranged in one or more layers (for example up to 48 tubes), areenvisaged.

The optical elements may be arranged into the inner tube 901 in acontinuous or in an open helix (S-Z) pattern around the axis of thecable.

Around the buffer tube 901 a water blocking tape 905 as previouslydescribed can be wound in a helical lay. A reinforcing layer 906 can bedisposed around the waterblocking tape and an outer polyethylene sheath907 is then disposed to surround the cable structure.

One or more reinforcing members 908 arranged longitudinally along thecable are inserted in the thickness of the said outer tubular sheath907. In one preferred embodiment, as illustrated in FIG. 3, tworeinforcing members 908 are present, advantageously arrangeddiametrically opposite each other. In addition, a reinforcing member canbe alternatively or additionally placed inside the inner tube 901 in anaxial position.

These members are preferably completely immersed in the said sheath andpreferably consist of reinforcing rods of high-strength material,typically between 0.5 and 2.5 mm in size. Said reinforcing members canbe made of a composite material, such as glass resin or reinforcedcarbon fiber resin or aramide yarns (Kevlar®), or alternatively of ametallic material such as steel and the like.

Alternatively, the tube 901 can be omitted and thus a single tubularpolymeric sheath 907 can carry out the twofold function of an outerprotective sheath and an inner tube.

Radiation-curable carrier systems which are suitable for forming acomposition to be used as primary coating in an optical fiber accordingto the invention contain one or more radiation-curable oligomers ormonomers (reactive diluents) having at least one functional groupcapable of polymerization when exposed to actinic radiation. Suitableradiation-curable oligomers or monomers are now well known and withinthe skill of the art. Commonly, the radiation-curable functionality usedis ethylenic unsaturation, which can be polymerized preferably throughradical polymerization. Preferably, at least about 80 mole %, morepreferably, at least about 90 mole %, and most preferably substantiallyall of the radiation-curable functional groups present in the oligomerare acrylate or methacrylate. For the sake of simplicity, the term“acrylate” as used throughout the present application covers bothacrylate and methacrylate functionality.

A primary coating according to the present invention can be made from aradiation curable coating composition comprising a radiation curableoligomer, said oligomer comprising a backbone derived from polypropyleneglycol and a dimer acid based polyester polyol. Preferably, the oligomeris a urethane acrylate oligomer comprising said backbone, morepreferably a wholly aliphatic urethane acrylate oligomer. Said oligomeris disclosed, for instance, in WO 01/05724. The oligomer can be madeaccording to methods that are well known in the art. Preferably, theurethane acrylate oligomer can be prepared by reacting

(A1) the polypropylene glycol, and

(A2) the dimer acid based polyester polyol,

(B) a polyisocyanate, and

(C) a (meth)acrylate containing a hydroxyl group.

Given as examples of the process for manufacturing the urethane acrylateby reacting these compounds are

(i) reacting said glycol (A1 and A2), the polyisocyanate, and thehydroxyl group-containing (meth)acrylate altogether; or

(ii) reacting said glycol and the polyisocyanate, and reacting theresulting product with the hydroxyl group-containing (meth)acrylate; or

(iii) reacting the polyisocyanate and the hydroxyl group-containing(meth)acrylate, and reacting the resulting product with said glycol; or

(iv) reacting the polyisocyanate and the hydroxyl group-containing(meth)acrylate, reacting the resulting product with said glycol, andreacting the hydroxyl group-containing (meth)acrylate once more.

Polypropylene glycol (A1)—as used herein—is understood to refer to apolypropylene glycol comprising composition having a plurality ofpolypropylene glycol moieties. Preferably, said polypropylene glycol hason average a number average molecular weight ranging from 1,000 to13,000, more preferably ranging from 1,500 to 8,000, even more preferredfrom 2,000 to 6,000, and most preferred from 2,500 to 4,500. Accordingto a preferred embodiment, the amount of unsaturation (referred to themeq/g unsaturation for the total composition) of said polypropyleneglycol is less than 0.01 meq/g, more preferably between 0.0001 and 0.009meq/g.

Polypropylene glycol includes 1,2-polypropylene glycol,1,3-polypropylene glycol and mixtures thereof, with 1,2-polypropyleneglycol being preferred. Suitable polypropylene glycols are commerciallyavailable under the trade names of, for example, Voranol P1010, P 2001and P 3000 (supplied by Dow), Lupranol 1000 and 1100 (supplied byElastogran), ACCLAIM 2200, 3201, 4200, 6300, 8200, and Desmophen 1111BD, 1112 BD, 2061 BD, 2062 BD (all manufactured by Bayer), and the like.Such urethane compounds may be formed by any reaction technique suitablefor such purpose.

Dimer acid based polyester polyol (A2)—as used herein—is understood torefer to a hydroxyl-terminated polyester polyol which has been made bypolymerizing an acid-component and a hydroxyl-component and which hasdimer acid residues in its structure, wherein said dimer acid residuesare residues derived from the use of a dimer acid as at least part ofthe acid-component and/or by the use of the diol derivative of a dimeracid as at least part of the hydroxyl-component.

Dimer acids (and esters thereof) are a well known commercially availableclass of dicarboxylic acids (or esters). They are normally prepared bydimerizing unsaturated long chain aliphatic monocarboxylic acids,usually of 13 to 22 carbon atoms, or their esters (e.g. alkyl esters).The dimerization is thought by those in the art to proceed by possiblemechanisms which include Diels-Alder, free radical, and carbonium ionmechanisms. The dimer acid material will usually contain 26 to 44 carbonatoms. Particularly, examples include dimer acids (or esters) derivedfrom C-18 and C-22 unsaturated monocarboxylic acids (or esters) whichwill yield, respectively, C-36 and C-44 dimer acids (or esters). Dimeracids derived from C-18 unsaturated acids, which include acids such aslinoleic and linolenic are particularly well known (yielding C-36 dimeracids).

The dimer acid products will normally also contain a proportion oftrimer acids (e.g. C-54 acids when using C-18 starting acids), possiblyeven higher oligomers and also small amounts of the monomer acids.Several different grades of dimer acids are available from commercialsources and these differ from each other primarily in the amount ofmonobasic and trimer acid fractions and the degree of unsaturation.

Usually the dimer acid (or ester) products as initially formed areunsaturated which could possibly be detrimental to their oxidativestability by providing sites for crosslinking or degradation, and soresulting in changes in the physical properties of the coating filmswith time. It is therefore preferable (although not essential) to usedimer acid products which have been hydrogenated to remove a substantialproportion of the unreacted double bonds.

Herein the term “dimer acid” is used to collectively convey both thediacid material itself or ester-forming derivatives thereof (such aslower alkyl esters) which would act as an acid component in polyestersynthesis and includes (if present) any trimer or monomer.

The dimer acid based polyester polyol preferably has on average a numberaverage molecular weight ranging from 1,000 to 13,000, more preferablyranging from 1,500 to 8,000, even more preferred from 2,000 to 6,000,and most preferred from 2,500 to 4,000.

Examples of these dimer acid based polyester polyols are given in EP 0539 030 B1 which polyols are incorporated herein by reference. Ascommercially available products, Priplast 3190, 3191, 3192, 3195, 3196,3197, 3198, 1838, 2033 (manufactured by Uniqema), and the like can begiven.

The ratio of polypropylene glycol to dimer acid based polyester polyolin the oligomer may be ranging from 1:5 to 5:1, preferably ranging from1:4 to 4:1, and more preferably ranging from 1:2 to 2:1, even morepreferably, polypropylene glycol and dimer acid based polyester polyolare present in an equimolar ratio.

Given as examples of the polyisocyanate (B) are 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 1,3-xylylene diisocyanate,1,4-xylylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylenediisocyanate, p-phenylene diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethanediisocyanate, 3,3′-dimethylphenylene diisocyanate, 4,4′-biphenylenediisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate,methylenebis(4-cyclohexylisocyanate), 2,2,4-trimethylhexamethylenediisocyanate, bis(2-isocyanatethyl)fumarate, 6-isopropyl-1,3-phenyldiisocyanate, 4-diphenylpropane diisocyanate, hydrogenateddiphenylmethane diisocyanate, hydrogenated xylylene diisocyanate,tetramethyl xylylene diisocyanate, lysine isocyanate, and the like.These polyisocyanate compounds may be used either individually or incombinations of two or more. Preferred isocyanates are tolylenedi-isocyanate, isophorone di-isocyanate, and methylene-bis(4-cyclohexylisocyanate). Most preferred are wholly aliphatic basedpolyisocyanate compounds, such as isophorone di-isocyanate, andmethylene-bis (4-cyclohexyl isocyanate).

Examples of the hydroxyl group-containing acrylate (C) include,(meth)acrylates derived from (meth)acrylic acid and epoxy and(meth)acrylates comprising alkylene oxides, more in particular,2-hydroxyethyl(meth)acrylate, 2-hydroxypropylacrylate and2-hydroxy-3-oxyphenyl(meth)acrylate. Acrylate functional groups arepreferred over methacrylates.

The ratio of the polyol (A) [said polyol (A) comprising (A1) and (A2)],the polyisocyanate (B), and the hydroxyl group-containing acrylate (C)used for preparing the urethane acrylate is determined so that 1.1 to 3equivalents of an isocyanate group included in the polyisocyanate and0.1 to 1.5 equivalents of a hydroxyl group included in the hydroxylgroup-containing (meth)acrylate are used for one equivalent of thehydroxyl group included in the polyol.

The number average molecular weight of the urethane (meth)acrylateoligomer used in the composition of the present invention is preferablyin the range from 1200 to 20,000, and more preferably from 2,200 to10,000. If the number average molecular weight of the urethane(meth)acrylate is less than 100, the resin composition tends tosolidify; on the other hand, if the number average molecular weight islarger than 20,000, the viscosity of the composition becomes high,making handling of the composition difficult.

The urethane (meth)acrylate oligomer is preferably used in an amountfrom 10 to 90 wt %, more preferably from 20 to 80 wt %, even morepreferably from 30 to 70 wt. %, and most preferred from 40 to 70 wt. %of the total amount of the resin composition. When the composition isused as a coating material for optical fibers, the range from 20 to 80wt. % is particularly preferable to ensure excellent coatability, aswell as superior flexibility and long-term reliability of the curedcoating.

A radiation-curable composition according to the invention may alsocontain one or more reactive diluents (B) that are used to adjust theviscosity. The reactive diluent can be a low viscosity monomer having atleast one functional group capable of polymerization when exposed toactinic radiation. This functional group may be of the same nature asthat used in the radiation-curable oligomer. Preferably, the functionalgroup of each reactive diluent is capable of copolymerizing with theradiation-curable functional group present on the otherradiation-curable diluents or oligomer. The reactive diluents used canbe mono- and/or multifunctional, preferably (meth)acrylate functional.

A suitable radiation-curable primary coating composition comprises fromabout 1 to about 80 wt. % of at least one radiation-curable diluent.Preferred amounts of the radiation-curable diluent include from about 10to about 60 wt. %, more preferably from about 20 to about 55 wt. %, evenmore preferred ranging from 25 to 40 wt. %, based on the total weight ofthe coating composition.

Generally, each reactive diluent has a molecular weight of less thanabout 550 and a viscosity of less than about 500 mPa·s

For example, the reactive diluent can be a monomer or a mixture ofmonomers having an acrylate or vinyl ether functionality and a C₄-C₂₀alkyl or polyether moiety. Examples of acrylate functionalmonofunctional diluents are acrylates containing an alicyclic structuresuch as isobornyl acrylate, bornyl acrylate, dicyclopentanyl acrylate,cyclohexyl acrylate, and the like, 2-hydroxyethyl acrylate,2-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, methyl acrylate,ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate,amyl acrylate, isobutyl acrylate, t-butyl acrylate, pentyl acrylate,isoamyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate,isooctyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decylacrylate, isodecyl acrylate, undecyl acrylate, dodecyl acrylate, laurylacrylate, stearyl acrylate, isostearyl acrylate, tetrahydrofurfurylacrylate, butoxyethyl acrylate, ethoxydiethylene glycol acrylate,benzylacrylate, phenoxyethylacrylate, polyethylene glycol monoacrylate,polypropylene glycol monoacrylate, methoxyethylene glycol acrylate,ethoxyethyl acrylate, methoxypolyethylene glycol acrylate,methoxypropylene glycol acrylate, dimethylaminoethyl acrylate,diethylaminoethyl acrylate, 7-amino-3,7-dimethyloctyl acrylate, acrylatemonomers shown by the following formula (1),

wherein R⁷ is a hydrogen atom or a methyl group, R⁸ is an alkylene grouphaving 2-6, and preferably 2-4 carbon atoms, R⁹ is a hydrogen atom or anorganic group containing 1-12 carbon atoms or an aromatic ring, and r isan integer from 0 to 12, and preferably from 1 to 8.

Of these, in order to obtain a cured polymeric material having asuitably low hardening temperature and a suitably low modulus at saidtemperature, long aliphatic chain-substituted monoacrylates, such as,for example decyl acrylate, isodecyl acrylate, tridecyl acrylate, laurylacrylate, and the like, are preferred and alkoxylated alkyl phenolacrylates, such as ethoxylated and propoxylated nonyl phenol acrylateare further preferred.

Examples of non-acrylate functional monomer diluents areN-vinylpirrolidone, N-vinyl caprolactam, vinylimidazole, vinylpyridine,and the like.

These N-vinyl monomers preferably are present in amounts between about 1and about 20% by weight, more preferably less than about 10% by weight,even more preferred ranging from 2 to 7% by weight.

According to a preferred embodiment, the polymeric material applied asprimary coating according to the invention is made from a radiationcurable composition comprising at least one monofunctional reactivediluent (having an acrylate or vinyl ether functionality), saidmonofunctional diluent(s) being present in amounts ranging from 10 to 50wt. %, preferably ranging from 20 to 40 wt. %, more preferably from 25to 38 wt. %. The amount of mono-acrylate functional reactive diluentspreferably ranges from 10 to 40 wt. %, more preferably from 15 to 35 wt.% and most preferred from 20 to 30 wt. %.

The reactive diluent can also comprise a diluent having two or morefunctional groups capable of polymerization. Examples of such monomersinclude: C₂-C₁₈ hydrocarbondiol diacrylates, C₄-C₁₈ hydrocarbondivinylethers,

C₃-C₁₈ hydrocarbon triacrylates, and the polyether analogues thereof,and the like, such as 1,6-hexanedioldiacrylate, trimethylolpropanetriacrylate, hexanediol divinylether, triethyleneglycol d iacrylate,pentaerythritol triacrylate, ethoxylated bisphenol-A diacrylate, andtripropyleneglycol diacrylate.

Such multifunctional reactive diluents are preferably (meth)acrylatefunctional, preferably difunctional (component (B1)) and trifunctional(component (B2)).

Preferably, alkoxylated aliphatic polyacrylates are used, such asethoxylated hexanedioldiacrylate, propoxylated glyceryl triacrylate orpropoxylated trimethylol propane triacrylate.

Preferred examples of diacrylates are alkoxylated aliphatic glycoldiacrylate, more preferably, propoxylated aliphatic glycol diacrylate. Apreferred example of a triacrylate is trimethylol propane triacrylate.

According to a preferred embodiment the polymeric material applied asprimary coating on an optical fiber according to the invention is madefrom a radiation curable which comprises, a multifunctional reactivediluent amounts ranging from 0.5-10 wt. %, more preferably ranging from1 to 5 wt. %, and most preferred from 1.5 to 3 wt. %.

Without being bound to any particular theory, the present inventorsbelieve that the combination of the oligomer according to the presentinvention in amounts of less than about 75 wt. % (preferably less thanabout 70 wt. %) with a total amount of monofunctional reactive diluentsof at least about 15 wt. % (more preferably, at least about 20 wt. %,even more preferably at least about 25 wt. % and most preferred at leastabout 30 wt. %) aids in achieving a primary coating composition, thatafter cure, has an acceptably low hardening temperature and low modulusat said temperature.

It is further preferred that the composition comprises a mixture of atleast two monofunctional reactive diluents, more preferably, one of saidreactive diluents being substituted with a long aliphatic chain; evenmore preferably, the composition contains two long aliphaticchain-substituted monoacrylates. Preferably, at least about 10 wt. %,more preferably at least about 12 wt. % is present of said at least onelong aliphatic chain-substituted monoacrylate.

A liquid curable resin composition suitable to be applied as a primarycoating layer on an optical fiber according to the present invention canbe cured by radiation. Here, radiation includes infrared radiation,visible rays, ultraviolet radiation, X-rays, electron beams, α-rays,β-rays, γ-rays, and the like. Visible and UV radiation are preferred.

The liquid curable resin composition suitable to be applied as a primarycoating layer on an optical fiber according to the present inventionpreferably comprises a photo-polymerization initiator. In addition, aphotosensitizer can added as required. Given as examples of thephoto-polymerization initiator are 1-hydroxycyclohexylphenyl ketone,2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde,fluorene, anthraquinone, triphenylamine, carbazole,3-methylacetophenone, 4-chlorobenzophenone, 4,4′-dimethoxybenzophenone,4,4′-diaminobenzophenone, Michler's ketone, benzoin propyl ether,benzoin ethyl ether, benzyl methyl ketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanethone,diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,bis-(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and the like.

Examples of commercially available products of the photo-polymerizationinitiator include IRGACURE 184, 369, 651, 500, 907, CGI1700, 1750, 1850,819, Darocur I116, 1173 (manufactured by Ciba Specialty Chemicals Co.,Ltd.), Lucirin LR8728 (manufactured by BASF), Ubecryl P36 (manufacturedby UCB), and the like.

The amount of the polymerization initiator used can range from 0.1 to 10wt %, and preferably from 0.5 to 7 wt %, of the total amount of thecomponents for the resin composition.

In addition to the above-described components, various additives such asantioxidants, UV absorbers, light stabilizers, silane coupling agents,coating surface improvers, heat polymerization inhibitors, levelingagents, surfactants, colorants, preservatives, plasticizers, lubricants,solvents, fillers, aging preventives, and wettability improvers can beused in the liquid curable resin composition of the present invention,as required. Examples of antioxidants include Irganox 1010, 1035, 1076,1222 (manufactured by Ciba Specialty Chemicals Co., Ltd.), Antigene P,3C, FR, Sumilizer GA-80 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of UV absorbers include Tinuvin P,234, 320, 326, 327, 328, 329, 213 (manufactured by Ciba SpecialtyChemicals Co., Ltd.), Seesorb 102, 103, 110, 501, 202, 712, 704(manufactured by Sypro Chemical Co., Ltd.), and the like; examples oflight stabilizers include Tinuvin 292, 144, 622LD (manufactured by CibaSpecialty Chemicals Co., Ltd.), Sanol LS770 (manufactured by Sankyo Co.,Ltd.), Sumisorb TM-061 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of silane coupling agents includeaminopropyltriethoxysilane, mercaptopropyltrimethoxy-silane, andmethacryloxypropyltrimethoxysilane, and commercially available productssuch as SH6062, SH6030 (manufactured by Toray-Dow Corning Silicone Co.,Ltd.), and KBE903, KBE603, KBE403 (manufactured by Shin-Etsu ChemicalCo., Ltd.).

The viscosity of the liquid curable resin composition applied as aprimary coating layer on an optical fiber according to the presentinvention is usually in the range from 200 to 20,000 cP, and preferablyfrom 2,000 to 15,000 cP.

The primary coating compositions suitable to be applied as a primarycoating layer on an optical fiber according to the present invention,when cured, typically have an elongation-at-break of greater than 80%,more preferably of at least 110%, more preferably at least 150% but nottypically higher than 400%.

The compositions suitable to be applied as a primary coating layer on anoptical fiber according to the present invention will preferably have acure speed of 1.0 J/cm² (at 95% of maximum attainable modulus) or less,more preferably about 0.7 J/cm² or less, and more preferably, about 0.5J/cm² or less, and most preferred, about 0.4 J/cm² or less.

An optical fiber according to the invention comprises a second layer ofpolymeric material (secondary coating) which is disposed to surroundsaid primary coating. Preferably, the polymeric material of saidsecondary coating is also based on a radiation curable composition. Theaforedescribed primary coating is then in turn coated with a secondarycoating, of a type known in the art, compatible with the primary coatingformulation. For example, if the primary coating has an acrylic base,the secondary coating will also preferably have an acrylic base.

Typically, an acrylic based secondary coating comprises at least oneoligomer with acrylate or methacrylate terminal groups, at least oneacrylic diluent monomer and at least one photoinitiator.

The oligomer represents generally 40-80% of the formulation by weight.The oligomer commonly consists of a polyurethaneacrylate.

The polyurethaneacrylate is prepared by reaction between a polyolstructure, a polyisocyanate and a monomer carrying the acrylic function.

The molecular weight of the polyol structure is indicatively between 500and 6000 u.a.; it can be entirely of hydrocarbon, polyether, polyester,polysiloxane or fluorinated type, or be a combination thereof. Thehydrocarbon and polyether structure and their combinations arepreferred. A structure representative of a polyether polyol can be, forexample, polytetramethylene oxide, polymethyltetramethylene oxide,polymethylene oxide, polypropylene oxide, polybutylene oxide, theirisomers and their mixtures. Structures representative of a hydrocarbonpolyol are polybutadiene or polyisobutylene, completely or partlyhydrogenated and functionalized with hydroxyl groups.

The polyisocyanate can be of aromatic or aliphatic type, such as, forinstance, a polyisocyanate (B) as previously described.

The monomer carrying the acrylic function comprises groups able to reactwith the isocyanic group. Said monomer can be selected, for instance,among the hydroxyl group-containing acrylates (C) as previouslyillustrated.

The epoxyacrylate is prepared by reacting the acrylic acid with aglycidylether of an alcohol, typically bisphenol A or bisphenol F.

The diluent monomer represents 20-50% of the formulation by weight, itsmain purpose being to cause the formulation to attain a viscosity ofabout 5 Pas at the secondary coating application temperature. Thediluent monomer, carrying the reactive function, preferably of acrylictype, has a structure compatible with that of the oligomer. The acrylicfunction is preferred. The diluent monomer can contain an alkylstructure, such as isobornylacrylate, hexanediacrylate,dicyclopentadiene-acrylate, trimethylolpropane-triacrylate, or aromaticsuch as nonylphenyletheracrylate, polyethyleneglycol-phenyletheracrylateand acrylic derivatives of bisphenol A.

A photoinitiator, such as those previously illustrated is preferablyadded to the composition. Further additives, such as inhibitorsinhibiting polymerization by the effect of temperature, lightstabilizers, leveling agents and detachment promoters can also be added

A typical formulation of a cross-linkable system for secondary coatingscomprises about 40-70% of polyurethaneacrylate, epoxyacrylate or theirmixtures, about 30-50% of diluent monomer, about 1-5% of photoinitiatorand about 0.5-5% of other additives.

An example of a formulation usable as the secondary coating of theinvention is that marketed under the name of DeSolite® 3471-2-136 (DSM).The fibers obtained thereby can be used either as such within opticalcables, or can be combined, for example in ribbon form, by incorporationinto a common polymer coating, of a type known in the art (such asCablelite® 3287-9-53, DSM), to be then used to form an optical cable.

Typically, the polymeric material forming the secondary coating has amodulus E′ at 25° C. of from about 1000 MPa to about 2000 MPa and aglass transition temperature (measured as above defined) higher thanabout 30° C., preferably higher than 40° C. and more preferably higherthan about 50° C.

An optical fiber according to the present invention may be producedaccording to the usual drawing techniques, using, for example, a systemsuch as the one schematically illustrated in FIG. 2.

This system, commonly known as “drawing tower,” typically comprises afurnace (302) inside which a glass optical preform to be drawn isplaced. The bottom part of the said preform is heated to the softeningpoint and drawn into an optical fiber (301). The fiber is then cooled,preferably to a temperature of at least 60° C., preferably in a suitablecooling tube (303) of the type described, for example, in patentapplication WO 99/26891, and passed through a diameter measurementdevice (304). This device is connected by means of a microprocessor(313) to a pulley (310) which regulates the spinning speed; in the eventof any variation in the diameter of the fiber, the microprocessor (313)acts to regulate the rotational speed of the pulley (310), so as to keepthe diameter of the optical fiber constant. Then, the fiber passesthrough a primary coating applicator (305), containing the coatingcomposition in liquid form, and is covered with this composition to athickness of about 25 μm-35 μm. The coated fiber is then passed througha UV oven (or a series of ovens) (306) in which the primary coating iscured. The fiber coated with the cured primary coating is then passedthrough a second applicator (307), in which it is coated with thesecondary coating and then cured in the relative UV oven (or series ofovens) (308). Alternatively, the application of the secondary coatingmay be carried out directly on the primary coating before the latter hasbeen cured, according to the “wet-on-wet” technique. In this case, asingle applicator is used, which allows the sequential application ofthe two coating layers, for example, of the type described in patentU.S. Pat. No. 4,474,830. The fiber thus coated is then cured using oneor more UV ovens similar to those used to cure the individual coatings.

Subsequent to the coating and to the curing of this coating, the fibermay optionally be caused to pass through a device capable of giving apredetermined torsion to this fiber, for example of the type describedin international patent application WO 99/67180, for the purpose ofreducing the PMD (“Polarization Mode Dispersion”) value of this fiber.The pulley (310) placed downstream of the devices illustrated previouslycontrols the spinning speed of the fiber. After this drawing pulley, thefiber passes through a device (311) capable of controlling the tensionof the fiber, of the type described, for example, in patent applicationEP 1 112 979, and is finally collected on a reel (312).

An optical fiber thus produced may be used in the production of opticalcables. The fiber may be used either as such or in the form of ribbonscomprising several fibers combined together by means of a commoncoating.

EXAMPLES

The present invention will be explained in more detail below by way ofexamples, which are not intended to be limiting of the presentinvention.

Coating Compositions

Coating compositions have been prepared to be applied as primary coatingon optical fibers. The compositions to be applied as a primary coatingon an optical fiber according to the invention are indicated as Ex. 1,Ex. 2 and Ex. 3 in the following table 1. TABLE 1 Radiation curableprimary coating compositions Ex. 1 Ex. 2 Ex. 3 (Wt. %) (Wt. %) (Wt. %)Oligomer I 68.30 60.30 67.30 Ethoxylated nonyl phenol acrylate 10.0019.00 10.00 Tridecyl acrylate 10.00 10.00 10.00 Long aliphaticchain-substituted 2.00 2.00 2.00 monoacrylate Vinyl caprolactam 5.006.00 5.00 Ethoxylated bisphenol A diacrylate 1.00 — 3.00 Trimethylolpropane triacrylate (TMPTA) 1.00 — — 2,4,6-trimethylbenzoyl diphenylphosphine 1.40 1.40 1.40 oxide Thiodiethylene bis[3-(3,5-di-tert-butyl-4- 0.30 0.30 0.30 hydroxyphenyl) propionate])hydrocinnamate γ-mercapto propyl trimethoxysilane 1.00 1.00 1.00

Oligomer I is the reaction product of isophorone diisocyanate (IPDI),2-hydroxyethylacrylate (HEA), polypropylene glycol (PPG) and a dimeracid based polyester polyol.

In addition, comparative commercial primary coating DeSolite® 3471-1-129(Comp. A in table 2) has also been tested.

The equilibrium modulus, the Tg, the Th and the modulus at the Th foreach of the above cured primary coating compositions were as given inTable 2 (see test method section for details on DMA test anddetermination of respective parameters on the DMA curve). Thecorresponding DMA curves of said cured coating compositions are reportedin FIGS. 4A to 4C, respectively. TABLE 2 Parameters of cured primarycoating compositions Ex. Tg Th E′ E′ (Th) 1 −59.1 −12.2 1.1 3.5 2 −56.6−10.8 0.7 2.0 3 −63.2 −13.3 1.1 2.7 Comp. A −55.1 −5.6 1.9 3.6

Test Methods and Methods of Manufacturing

Curing of the Primary Coatings for Mechanical Testing (SamplePreparation)

A drawdown of the material to be tested was made on a glass plate andcured using a UV processor in inert atmosphere (with a UV dose of 1J/cm², Fusion D-lamp measured with EIT Uvicure or International Light IL390 B Radiometer). The cured film was conditioned at 23±2° C. and 50±5%RH for a minimum of 16 hours prior to testing.

A minimum of 6 test specimens having a width of 12.7 mm and a length of12.5 cm were cut from the cured film.

Dynamic Mechanical Testing

The DMA testing has been carried out in tension according to thefollowing methodology.

Test samples of the cured coating compositions of examples 1-3 and ofcomparative experiment A were measured using a Rheometrics SolidsAnalyzer (RSA-11), equipped with:

1) a personal computer having a Windows operating system and having RSIOrchestrator® software (Version V.6.4.1) loaded, and

2) a liquid nitrogen controller system for low-temperature operation.

The test samples were prepared by casting a film of the material, havinga thickness in the range of 0.02 mm to 0.4 mm, on a glass plate. Thesample film was cured using a UV processor. A specimen approximately 35mm (1.4 inches) long and approximately 12 mm wide was cut from adefect-free region of the cured film. For soft films, which tend to havesticky surfaces, a cotton-tipped applicator was used to coat the cutspecimen with talc powder.

The film thickness of the specimen was measured at five or morelocations along the length. The average film thickness was calculated to±0.001 mm. The thickness cannot vary by more than 0.01 mm over thislength. Another specimen was taken if this condition was not met. Thewidth of the specimen was measured at two or more locations and theaverage value calculated to ±0.1 mm.

The geometry of the sample was entered into the instrument. The lengthfield was set at a value of 23.2 mm and the measured values of width andthickness of the sample specimen were entered into the appropriatefields.

Before conducting the temperature sweep, moisture was removed from thetest samples by subjecting the test samples to a temperature of 80° C.in a nitrogen atmosphere for 5 minutes. The temperature sweep usedincluded cooling the test samples to about −60° C. or about −90° C. andincreasing the temperature at about 2° C./minute until the temperaturereached about 100° C. to about 120° C. The test frequency used was 1.0radian/second. In a DMTA measurement, which is a dynamic measurement,the following moduli are measured: the storage modulus E′ (also referredto as elastic modulus), and the loss modulus E″ (also referred to asviscous modulus). The lowest value of the storage modulus E′ in the DMTAcurve in the temperature range between 10 and 100° C. measured at afrequency of 1.0 radian/second under the conditions as described indetail above is taken as the equilibrium modulus of the coating.

The corresponding DMA curves are reported in FIGS. 4 a to 4 c (examples1-3 respectively) and FIG. 5 (comp. Exp. A).

Determination of Glass Transition Temperature (Tg) and HardeningTemperature (Th)

Based on the respective DMA plot of each cured primary coating material,the Tg, Th and modulus at Th of the material have been determined asmentioned in the descriptive part.

Thus, with ref. to FIG. 1, the Tg is determined by the intersectionpoint of line A with line D. Line A is determined by interpolating thepoints of the DMA curve in the plateau region of the glassy state in thefollowing manner. First of all, the median value of logE′ in the regionfrom −60° C. to −80° C. is calculated. Line A is then determined as thehorizontal line (parallel to the x axis) passing through said value ofLogE′. Line D is determined as the tangent to the inflection point ofthe DMA curve in the oblique portion “d” of said curve. The inflectionpoint and the inclination of the tangent in that point are determined bymeans of the first derivative of the DMA curve; the abscissa of theminimum point of the derivative curve gives the respective abscissa ofthe inflection point on the DMA curve, while the ordinate gives theinclination (angular coefficient) of the tangent line in said inflectionpoint. The derivative curve has been determined by calculating thederivative of each experimental point of the DMA curve and then fittingthese points by means of a 6^(th) degree polynomial curve in the range+20/−40° C. around the minimum calculated derivative points.

Similarly, also the Th has been determined as the intersection point ofline B with line D (see FIG. 1). Line D is as above determined, whileline B is determined by interpolating the points of the DMA curve in theplateau region of the rubbery state in the following manner. First ofall, the median value of logE′ in the region from 20° C. to 40° C. iscalculated. Line B is then determined as the horizontal line (parallelto the x axis) passing through said median value of LogE′.

Manufacturing of Optical Fibers

All the optical fibers used in the present experimental section has beenmanufactured according to standard drawing techniques, by applying afirst (primary) coating composition on the drawn optical fiber, curingsaid coating composition and subsequently applying the secondary coatinglayer and curing it. The fiber is drawn at a speed of about 20 m/s andthe cure degree of the coating layers is of at least 90%. The curedegree is determined by means of MICRO-FTIR technique, by determiningthe percentage of the reacted acrylate instaurations in the finalcross-linked resin with respect to the initial photo-curable composition(e.g. as described in WO 98/50317).

Microbending Tests

Microbending effects on optical fibers were determined by the“expandable drum method” as described, for example, in G. Grasso and F.Meli “Microbending losses of cabled single-mode fibers”, ECOC '88, pp.526-ff, or as defined by IEC standard 62221 (Optical fibers—Measurementmethods—Microbending sensitivity—Method A, Expandable drum; October2001). The test is performed by winding a 100 m length fiber with atension of 55 g on a 300 mm diameter expandable metallic bobbin, coatedwith rough material (3M Imperial® PSA-grade 40 μm).

The bobbin is connected with a personal computer which controls:

-   -   the expansion of the bobbin (in terms of variation of fiber        length); and    -   the fiber transmission loss.

The bobbin is then gradually expanded while monitoring fibertransmission loss versus fiber strain.

The pressure exerted onto the fiber is calculated from the fiberelongation by the following formula: $p = \frac{{EA}\quad ɛ}{R}$

where E is the elastic modulus of glass, A the area of the coated fiberand R the bobbin radius.

For each optical fiber, the MAC has been determined as follows:${MAC} = \frac{MFD}{\lambda_{co}}$where MFD (mode field diameter according Petermann definition) at 1550nm and λ_(CO) (lambda fiber cutoff −2 m length) are determined accordingto standard ITUT G650. Lambda cable cutoff (for NZD fibers) has beendetermined according to ITUT G650.

1. An optical fiber comprising: a) an internal glass portion; b) a firstcoating layer of a first polymeric material surrounding said glassportion; and c) a second coating layer of a second polymeric materialsurrounding said first coating layer, wherein said first polymericmaterial has a hardening temperature lower than −10° C., an equilibriumtensile modulus lower than 1.5 MPa, wherein said first coating layer hasa thickness of from 18 μm to 28 μm and wherein said second coating layerhas a thickness of from 10 μm to 20 μm.
 2. An optical fiber according toclaim 1 wherein the thickness of the first coating layer is 22-23 μm. 3.An optical fiber according to claim 1 or 2 wherein the thickness of thesecond coating layer is 15 μm.
 4. An optical fiber according to claim 1wherein said equilibrium tensile modulus is lower than 1.4 MPa.
 5. Anoptical fiber according to claim 1 wherein said equilibrium tensilemodulus is lower than 1.3 MPa.
 6. An optical fiber according to claim 1wherein the first polymeric material has an equilibrium tensile modulusnot lower than about 0.5 MPa.
 7. An optical fiber according to claim 1wherein the first polymeric material has an hardening temperature lowerthan −12° C.
 8. An optical fiber according to claim 1 wherein the firstpolymeric material has a glass transition temperature not higher thanabout −30° C.
 9. An optical fiber according to claim 8 wherein the glasstransition temperature is not higher than about −40° C.
 10. An opticalfiber according to claim 8 wherein the glass transition temperature isnot higher than about −50° C.
 11. An optical fiber according to claim 1wherein the first polymeric material has: d) a hardening temperature(Th) of from −10° C. to about −20° C. and a modulus measured at said Thlower than 5.0 MPa; or e) a hardening temperature (Th) of from −20° C.to about −30° C. and a modulus measured at said Th lower than 20.0 MPa;or f) a hardening temperature (Th) lower than about −30° C. and amodulus measured at said Th lower than 70.0 MPa.
 12. An optical fiberaccording to claim 1 wherein the first polymeric material has: a) ahardening temperature (Th) of from −10° C. to about −20° C. and amodulus measured at said Th lower than 4.0 MPa; or b) a hardeningtemperature (Th) of from −20° C. to about −30° C. and a modulus measuredat said Th lower than 15.0 MPa; or c) a hardening temperature (Th) lowerthan about −30° C. and a modulus measured at said Th lower than 50.0MPa.
 13. An optical fiber according to claim 1 wherein said secondpolymeric material has a modulus E′ at 25° C. of from about 1000 MPa toabout 2000 MPa.
 14. An optical fiber according to claim 1 wherein saidsecond polymeric material has a glass transition temperature higher thanabout 30° C.
 15. An optical fiber according to claim 14 wherein saidglass transition temperature is higher than 40° C.
 16. An optical fiberaccording to claim 14 wherein said glass transition temperature ishigher than about 50° C.
 17. An optical fiber according to claim 1wherein said first polymeric material is obtained by curing a radiationcurable composition comprising a radiation curable oligomer comprising abackbone derived from polypropylene glycol and a dimer acid basedpolyester polyol.